In general, a spark plug used for igniting an internal combustion engine such an automotive engine includes a tubular metallic shell; a tubular insulator disposed in the bore of the metallic shell; a center electrode disposed in a forward end portion of the axial hole of the insulator; a metallic terminal disposed in a rear end portion of the axial hole; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end faces the center electrode so as to form a spark discharge gap. Further, there has been known a spark plug including a resistor which is disposed in the axial hole between the center electrode and the metallic terminal so as to prevent generation of radio noise.
In general, such a resistor is formed from a mixture of glass powder and an electrically conductive substance such as carbon black powder or metal powder. However, since the amount of metal contained in the resistor is not so large, difficulty is encountered in joining the resistor directly to the metallic terminal and the center electrode, which are formed of metal. In order to eliminate the difficulty, in general, a seal layer containing a metal powder in a relatively large amount is disposed between the metallic terminal and the resistor and between the center electrode and the resistor to thereby increase the joint strength therebetween.
Incidentally, recent internal combustion engines for automobiles or the like have been required to produce a higher power and to operate with a higher efficiency, and development of a spark plug of a reduced size has been demanded in order to allow free design of engines and a reduction in the size of engines themselves. In order to reduce the size of a spark plug, the diameter of the insulator must be decreased. Meanwhile, in order to secure the desired characteristics such as mechanical strength of the insulator, the insulator must have a predetermined thickness in the radial direction. Accordingly, in order to reduce the diameter of the insulator, the diameter of the bore of the insulator must be decreased. However, when the diameter of the bore of the insulator is decreased, the diameters of the resistor and the seal layer also decrease. In such a case, the joint strength between the resistor and the seal layer decreases, and when an impact or vibration acts on the spark plug, a crack is generated at the interface between the resistor and the seal layer, which may result in deterioration of load life performance.
A spark plug which can solve such a problem is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-245716 (“Patent Document 1”). In claim 1 of Patent Document 1, there is recited a “spark plug characterized in that the diameter D of the electrically conductive glass seal layer is 3.3 mm or less, and the joint surface between the electrically conductive glass seal layer and the resistor is formed to have a curved shape.” Patent Document 1 states that, the invention can provide a “spark plug which is enhanced in adhesion between the resistor and the electrically conductive glass seal layer, which is excellent in vibration resistance and load life performance of the resistor, and which has a reduced diameter” (see paragraph 0012).